Research Journal of Applied Sciences, Engineering and Technology 6(14): 2621-2624, 2013
ISSN: 2040-7459; e-ISSN: 2040-7467
© Maxwell Scientific Organization, 2013
Submitted: January 05, 2013
Accepted: February 08, 2013
Published: August 10, 2013
Effect of Low Temperature Heat Treatment on Different Original Graphite
Microstructures of Nodular Cast Iron
Yan Zhao
Department of Mechanical and Electronical Engineering, Dezhou University, Dezhou 253023, China
Research Institute of Engineering Materials, Dezhou University, Dezhou 253023, China
Abstract: To understand the effect of low temperature heat treatment on microstructure of nodular graphite, nodular
cast irons with the same chemical composition but different microstructure were made by the heredity of raw
materials. The effects of low temperature heat treatment on the different microstructures of graphite were studied.
The results show that after the low temperature treatment, the modularization rate and level are improved observably
for as-cast graphite with lower degree of graphitization and worse effect of nodularization and the mean sphere
diameter of the graphite increases; the spot graphite arises for as-cast graphite with higher degree of graphitization
and better effect of nodularization, the mean sphere diameter of the graphite decreases, the spot and nodular graphite
distribute alternately, the spheroidization grade and nodularization rate do not change obviously.
Keywords: Low temperature heat treatment, microstructure of graphite, nodular graphite cast iron
INTRODUCTION
With the advent of nodular graphite cast iron since
1940s, it has been widely used in mechanical
engineering and other fields due to its predominant
performance and lower production cost. Along with the
rapid development of mechanical industry, the higher
performance of nodular graphite cast irons has been put
forward. In order to improve the microstructure and
performance of nodular graphite cast iron further,
scientific researchers have not only been limited to the
research areas of casting process and material
optimization, but also the effect of heat treatment,
especially the low temperature heat treatment has
attached a lot of academic attention increasingly by
virtue of its low energy consumption and low cost
(Hao, 2004).
The principal factors those affect the
microstructure and performance of nodular graphite
cast iron are microstructure of matrix, graphite’s shape,
size, quantity, distribution and so on (Gülcan et al.,
2006; Zhao et al., 2004; Torsten and Ingvar, 2007;
Nuno et al., 2008; Hao and Yang, 2008). At present,
researches about the effect of heat treatment on matrix
of nodular graphite cast iron are comparative maturity
(Chang et al., 2006, 2008; Martínez-Madrid et al.,
2002; Putatunda and Gadicherla, 2000; Delia et al.,
1998), but the reports about the effect of heat treatment
on microstructure of graphite are less.
In order to get a comprehensive understanding of
the effect of low temperature heat treatment on
microstructure of nodular graphite, at the same time
considering reducing the energy consumption and
production cost. The effects of low temperature heat
treatment on different original graphite structures of
nodular graphite cast irons are studied in this study.
METHODOLOGY
Five samples of nodular graphite cast iron with the
same chemical composition used for heat treatment in
this study were made by the heredity of raw materials in
our previous work (Zhao and Yang, 2009). The main
chemical compositions of raw materials for nodular
graphite cast irons production are listed in Table 1.
Schemes of mixture ratio are shown in Table 2. Under
the techniques of carbureting and silicon content
regulating, samples were casting into sand molds with a
size of Φ40×150 mm. The finally chemical composition
of nodular graphite cast irons is controlled as: C3.623.64%, Si 2.52-2.55%, Mn 0.23-0.50%, S≤0.015%,
P≤0.05% (Xie and Yang, 2007). The low temperature
heat treatment process of samples is holding at 700°C
for 2 h, then taken out air cooling. The microstructures
of samples were observed by the optical microscope
and high power video microscope HK-2200. The
changes of samples’ graphite microstructures after low
temperature heat treatment were analyzed comparing
with their original microstructures.
RESULTS AND DISCUSSION
Figure 1 shows as-cast graphite forms of each
nodular graphite cast iron. As can be seen from Fig. 1
that the nodular graphite cast irons with the same
chemical composition but different schemes of mixture
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Res. J. Appl. Sci. Eng. Technol., 6(14): 2621-2624, 2013
Table 1: Chemical composition of raw materials (mass fraction, %)
Composition
C
Si
Mn
Q12 pig iron
4.40
1.100
0.15
Scrap steel
0.52
0.315
0.55
Nodularizer
41.000
-
S
0.020
0.024
-
P
0.050
0.021
-
Mg
8.0
RE
4.5
Ca
2.6
Table 2: Different mixture ratio of pig iron and scrap steel (mass
fraction ratio)
Samples
1#
2#
3#
4#
5#
Pig iron: scrap steel
4:1
3:2
2:3
1:4
0:5
Fig. 1: As-cast graphite microstructures of nodular graphite
cast irons made by different mixture ratio (a) 1#
sample, (b) 2# sample, (c) 3# sample, (d) 4# sample, (e)
5# sample
Fig. 2: As
low
temperature
heat-treated
graphite
microstructures of nodular graphite cast irons made by
different mixture ratio (a) 1# sample, (b) 2# sample, (c)
3# sample, (d) 4# sample, (e) 5# sample
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Res. J. Appl. Sci. Eng. Technol., 6(14): 2621-2624, 2013
ratio have different texture characteristics due to the
heredity of raw materials.
The graphitization degree and nodularization effect
are not too well for sample 1# and 4#. The proportion of
graphite is less with a 3rd spheroidization grade, a 80%
nodularization rate and a 4.5 grade of graphite size.
Sample 2# and 3# are less affected by the heredity of
raw materials, so the graphitization degree and
nodularization effect are improved obviously. The
graphite are numerous and fine with a 1st-2nd
spheroidization grade, a 90-95% nodularization rate and
a 5.5 grade of graphite size. Sample 5# is affected the
most by the heredity of raw materials, the graphite are
few and far between and present spherical and
vermiform with a 5th spheroidization grade, a 65%
nodularization rate and a 4 grade of graphite size.
Figure 2 shows graphite microstructures of each
nodular graphite cast iron after the low temperature heat
treatment. As can be seen from Fig. 2 that the different
original graphite microstructures of nodular graphite
cast irons are affected diversely by the low temperature
heat
treatment.
The
nodularization
effects
(spheroidization grade and nodularization rate) of 1#, 4#
and 5# are improved obviously. Especially for sample
5#, the vermicular graphite trends to nodulizing. But the
graphite of the three samples grows up obviously
comparing with its original graphite microstructure
before the low temperature heat treatment. For samples
2# and 3#, there appear some spot graphite after the low
temperature heat treatment. And the average sphere
diameters of the graphite nodules decrease, but the
spheroidization grade and nodularization rate have no
obvious change.
The main reason for the above phenomenon is the
diffusion behavior of carbon atom in the low
temperature heat treatment process. One aspect is that
carbon atoms of the graphite nodule diffuse from the
outer surface of the graphite nodule to the matrix
gradually. On the other hand, carbon atoms dissolved in
the matrix diffuse from the matrix to the interfacial
zone of the graphite nodule and matrix and then
accumulate on the surface of the graphite nodule (Yao
et al., 2000).
As a result of the low heat treatment temperature
used in this study, eutectoid cementite in pearlite is
difficult to decompose and separate out graphite, which
can be found out from Fig. 3 (the matrix structure after
heat treatment) that the percentage of pearlite is almost
unchanged compared with the matrix structure before
heat treatment (Zhao and Yang, 2009).
Furthermore, it is impossible that carbon atoms
diffuse from the matrix to the interfacial zone of the
graphite nodule and matrix. So, the changes of the
graphite nodule mainly depend on carbon atoms’
diffusing from the outer surface of the graphite nodule
to the matrix and the concentration gradient of atoms is
the driving force for the diffusing of carbon atoms.
Because the graphitization degree and nodularization
effect of samples 1#, 4# and 5# before the low
temperature heat treatment are not satisfied, as shown
Fig. 3: As low temperature heat-treated matrix structures of
nodular graphite cast irons made by different mixture
ratio (a) 1# sample, (b) 2# sample, (c) 3# sample, (d) 4#
sample, (e) 5# sample
in Fig. 1a to e, therefore, the carbon atom content in the
matrix is relatively high. The less concentration
gradient of carbon atom between the matrix and
graphite nodule results in a slow diffusing from the
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Res. J. Appl. Sci. Eng. Technol., 6(14): 2621-2624, 2013
outer surface of the graphite nodule to the matrix.
Especially, the cooling rate is comparatively fast when
the air cooling is used, most of the carbon atoms cannot
diffuse to the matrix far away from the graphite nodule
and form a carbon-rich zone near the original graphite,
which results in the sphere diameter of the graphite
increasing. Simultaneously, to reduce the surface free
energy in the diffusing process of carbon atoms, the
base level outward spherical crystal comes into being.
Therefore, the nodularization effects are improved
obviously comparing with those before heat treatment.
Roundness of the nodular graphite is improved,
lumpish graphite transform into sphere, vermicular
graphite trends to nodulizing, as shown in Fig. 2a, d
and e. The spheroidization grade and nodularization
rate of sample 2# and 3# are very good before the heat
treatment. The graphite nodules are numerous and fine,
therefore, the carbon atom content in the matrix is very
low, which results in a comparatively large
concentration gradient of carbon atom between the
matrix and graphite nodule. This makes a portion of
carbon atoms potentially get away from the graphite
nodule and diffuse far away from the graphite to the
matrix. When abundant carbon atoms of some graphite
nodules diffused to the matrix, the carbon atom
concentration gradient of the nearby matrix reduces
certainly and prevents other surface carbon atoms of
graphite nodules from diffusing. As a result of the fast
diffusing of carbon atoms, the spot graphite partially
arises from sample 2# and 3# after the low temperature
heat treatment. On the whole, the mean sphere diameter
of the graphite decreases, the spot and nodular graphite
distribute alternately. At the same time, to get the lower
surface free energy, the graphite still keeps spherical
after the atomic diffusing process. Therefore, the
spheroidization grade and efficiency do not change
obviously before and after the low temperature heat
treatment.
CONCLUSION
•
•
•
The effects of low temperature heat treatment on
the different original graphite microstructures of
nodular graphite cast irons are diverse.
For as-cast graphite with lower degree of
graphitization and worse effect of nodularization,
the nodularization rate and spheroidization grade
are improved observably after the low temperature
treatment. Lumpish graphite transforms into
sphere, vermicular graphite trends to nodulizing,
but the mean sphere diameter of the graphite
increases.
For as-cast graphite with higher degree of
graphitization and better effect of nodularization,
the spheroidization grade and nodularization rate
do not change obviously after the low temperature
treatment. But the spot graphite arises, the mean
sphere diameter of the graphite decreases and the
spot and nodular graphite distribute alternately.
ACKNOWLEDGMENT
The authors would like to thank science and
technology program of Shandong province (Grant No.
2011YD03099).
REFERENCES
Chang, L.C., Z.C. Hsui and L.H. Chen, 2006. Effects of
heat treatment on the erosion behavior of
austempered ductile irons. Wear, 260: 783-793.
Chang, L.C., Z.C. Hsui and L.H. Chen, 2008. Influence
of austenization temperature on the erosion
behavior of austempered ductile irons. J. Univ.,
Sci. Technol. Beijing, 15(1): 29-31.
Delia, M., M. Alaalam and M. Grech, 1998. Effect of
austenitizing conditions on the impact properties of
an alloyed austempered ductile iron of initially
ferritic matrix structure. JMEPEG, 7: 265-272.
Gülcan, T., T. Mustafa and T. Alaaddin, 2006. Effect of
matrix structure on the impact properties of an
alloyed ductile iron. Mater. Charact., 57: 290-299.
Hao, S.J., 2004. Modern Cast Iron. Metallurgical
Industry Press, Beijing.
Hao, B.H. and M. Yang, 2008. Research on the
relationship between shape and intensity of
graphite ball. J. Beijing Inst. Petro-Chem.
Technol., 16(1): 39-43.
Martínez-Madrid, M., M.A. Acosta and A. TorresAcosta, 2002. Effects of austempering temperature
on fatigue crack rate propagation in a series of
modified (Cu, Ni and/or Mo) nodular irons.
JMEPEG, 11: 651-658.
Nuno, C., N. Machado and F.S. Silva, 2008. Influence
of graphite nodules geometrical features on fatigue
life of high-strength nodular cast iron. JMEPEG,
17: 352-362.
Putatunda, S.K. and P.K. Gadicherla, 2000. Effect of
austempering time on mechanical properties of a
low manganese austempered ductile iron.
JMEPEG, 9: 193-203.
Torsten, S.G. and L.S. Ingvar, 2007. The Effect of
graphite fraction and morphology on the plastic
deformation behavior of cast irons. Metall. Mater.
Trans. A, 38: 840-847.
Xie, R.C. and H. Yang, 2007. Effect of K/NA-RE
Multiple modificator on nodular Iron produced by
recaburation process. Hot Working Technol.,
36(9): 425-427.
Yao, X.Q., X.F. Shu and Z.B. Xing, 2000. Study on
growth mechanism of spherical graphite.
J. Kunming Univ., Sci. Technol., 25(5): 56-59.
Zhao, Y. and H. Yang, 2009. Effect of raw material
heredity on microstructure of nodular graphite cast
iron. Foundry, 58(5): 502-504.
Zhao, X., T.F. Jing and Y.W. Gao, 2004. Morphology
of graphite in hot-compressed nodular iron.
J. Mater. Sci., 39: 6093-6096.
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